1. Field of the Invention
The invention relates to a method for manufacturing ceramic foils from a castable slip made of a dispersion of a ceramic powder in a solvent.
2. Description of Related Art
Ceramic materials are being used more and more to produce articles which are thermally and chemically resistant, have a light weight or are even intended to be used as insulation. When producing the ceramic articles, first a green body is produced from a castable slip by drying, a xe2x80x9cwhite bodyxe2x80x9d being formed from the green body by baking thoroughly. The white bodies are sintered, resulting in a sintered compact of great strength which can be used, for example, in the field of information energy and electronic engineering.
To produce a castable slip, generally a ceramic powder is suspended in organic solvents together with necessary chemical additives such as liquefiers, binders and plasticizers. A dispersion results, from which a solid ceramic article is produced by drying, burning and sintering, the initial polymer bond being replaced by a polycrystalline ceramic bond. The polymers used for the dispersion are thermally decomposed during the thermal treatment and escape as a vapor or gas.
When producing the castable slip, selected polymers are dissolved with organic solvents and the ceramic powder is dispersed therein. In so doing, the ceramic powder is distributed as homogeneously as possible. The percentage of ceramic in the dispersion is between 30 and 85 percentage by weight, depending on the area of application and the desired properties of the ceramic body. The other portions of the dispersion include the polymer binder, the organic solvent, and possibly added auxiliary agents such as dispersing agents, defoaming agents and plasticizers which are intended to improve the castability and flexibility of the foils.
For example, to produce xe2x80x9cgreen foilsxe2x80x9d, the castable slip is poured out in as constant a thickness as possible onto a solid base. The slip is thereupon conveyed through a drying tunnel and the solvent is steamed out. The dispersion is thickened and dried, so that at the end of the drying tunnel, only the ceramic powder with the polymer and possibly intermingled auxiliary agents remains. The result is a flexible polymer foil having a very high filling ratio of ceramic powder.
One problem with ceramic foils produced in this manner is the use of the organic solvent. In light of environmental protection as well as explosion protection, they demand increased expenditure from the standpoint of process engineering. Thus, a number of organic solvents must not be released into the environment, making it necessary to design the drying installation as a recycling system, i.e., including filter equipment with subsequent waste disposal. In addition, because of the increased flammability and the low flame and explosion temperatures of the organic solvents, increased expenditure for equipment in the technical production installation is necessary, especially with respect to protection against explosion. The sometimes existing toxicity of the organic solvents also requires increased industrial safety for their processing and for the people coming in contact with them.
It is an object of the invention to avoid the above-indicated problems when producing ceramic bodies, particularly ceramic foils. This and other objects of the invention are achieved by a method for producing ceramic bodies, particularly ceramic foils, from a castable slip made of a dispersion of a ceramic powder in a solvent, wherein a dispersion of an aqueous vinyl-acetate homopolymer or an aqueous vinyl-acetate/ethene copolymer is used as a solvent.
Surprisingly, it was found that a solvent made of a dispersion of an aqueous vinyl-acetate homopolymer or an aqueous vinyl-acetate/ethene copolymer mixed with a ceramic powder yields a very good, highly homogeneous dispersion, from which green foils can be produced that are crack-free, non-porous and non-warping. The dispersion, based purely upon water, yields an environmentally friendly method without organic solvents and a versatilely usable binder component for the ceramic bodies. Furthermore, the vinyl-acetate/ethene copolymer dispersion in particular allows variable adjustment of the flexibility and strength of the foils to be produced. Thus, the monomer ratio of vinyl acetate to ethene can be varied within a wide range, preferably from 95:5 to 75:25. A higher portion of ethene yields a softer product, while a higher portion of vinyl acetate yields a harder product.
It has also turned out that the dispersion can be produced with a high solids content. Thus, it is possible to produce an aqueous dispersion having a polymer constituent of 5 to 25 percentage by weight relative to the total solids content. The ceramic content of the slip is very highly adjustable due to the low inherent viscosity of the dispersion, which results in short drying times.
It was also possible to ascertain that the green foils, or even the ceramic articles, produced according to the method of the present invention are re-emulsifiable. By re-wetting with water, a dispersion can be formed anew in which the ceramic is again distributed dispersively. This provides a possibility for recycling the ceramic powder utilized.
The special advantage of the polymer dispersion used is that, by varying its monomers, it can be adjusted to be variably flexible or brittle, mechanically strong or weak. The greater the quantity of powder, the stronger the product, i.e., both the monomer ratio and the ratio of powder quantity to polymer quantity determine the properties of the ceramic body in the final effect.
The basic concept of the invention, namely, the use of the special polymer for producing the dispersion, can be supplemented by additives. Thus, for example, it is possible to add a dispersant to the dispersion. Polyelectrolyte solutions or carboxylic acids in particular are advantageous for this purpose.
There is no restriction with respect to the ceramic powder used. Thus, siliceous, oxide or non-oxide ceramic powders can be employed. Their parts by weight in the dispersion is generally between 30 and 85 percentage by weight, however preferably over 70 percentage by weight. The non-oxide ceramic powders can be specially masked against hydrolysis. It is also possible to add a plasticizer to the dispersion, preferably between 1 and 15 percentage by weight relative to the solids content. Phthalates or polyhydric alcohols can be used as plasticizers.
The particle size of the dispersion is between 0.05 and 3.0 micrometers. Its transformation temperature lies between xe2x88x9240xc2x0 C. and +20xc2x0 C. To increase the flowability of the dispersion, a liquefier can be added in a quantity of 0.2 to 3.0 percentage by weight relative to the solids content.
The dispersion of the present invention is used primarily for producing ceramic foils, in that the dispersion is applied in uniform thickness onto a base with the aid of the doctor blade process, subsequently dried and sintered.